Differential current amplifier circuit

ABSTRACT

A differential current amplifier circuit having first and second transistors (Q1, Q2) whose collectors are supplied with first and second input signals (INA, INB), respectively, and first and second resistors (R1, R2) connected between the base and the collector of each of the transistors. When the resistors have the same resistance and the signals (INA, INB) are inputted to the input terminals, the potential of the common base node of the first and second transistors goes to a substantially ground potential, and the signal current component (i) does not flow through the first and second transistors. Thus, only DC currents (2I, 2nI) which are not superposed by the signal current component (i) flow through third and fourth transistors connected in a current mirror fashion, and an additional DC current source is not needed at this portion. Furthermore, the fourth transistor (Q4) is used as a constant current source of a differential pair constituted by fifth and sixth transistors (Q5, Q6), and an amplified signal component (2i 2 ) is obtained from the output terminal (OUT) of the differential pair. Thus, a large current gain is obtained.

TECHNICAL FIELD

The present invention relates to a differential current amplifiercircuit formed in a semiconductor integrated circuit.

BACKGROUND ART

A conventional differential current amplifier circuit, which is designedto amplify a differential input current, has the structure illustratedin FIG. 1. More specifically, the circuit comprises a first signal inputterminal IN1 and NPN transistors Q1 and Q2. The terminal IN1 isconnected to receive an input signal current (I+i) consisting of a DCcomponent I and a signal component (+i) superposed on the DC component.The collector-base path of the NPN transistor Q1 is connected to thefirst signal input terminal IN1 and the emitter of the NPN transistor Q1is connected to the ground potential GND. The NPN transistor Q2 has anemitter area n times greater than that of the transistor Q1 and isconnected to transistor Q2 in currentmirror fashion. The collector ofthe NPN transistor Q2 is connected to a first terminal OUT1. Thetransistors Q1 and Q2 form a first current mirror circuit. A secondsignal input terminal IN2 is connected to receive an input signalcurrent (I-i) consisting of a DC component I and a signal component (-i)which is differential to the signal component (+i) and superposed on theDC component I. The amplifier circuit further comprises NPN transistorsQ3 and Q4. The collector-base path of the NPN transistor Q3 is connectedto the second signal input terminal IN2, and the emitter of the NPNtransistor Q3 is connected to the ground potential GND. The transistorQ4 has an emitter area n times greater than that of the transistor Q3,and is connected to the transistor Q3 in current-mirror fashion. Thecollector of the transistor Q4 is connected to a second output terminalOUT2. The transistors Q3 and Q4 form a second current mirror circuit.

In the differential current amplifier circuit of FIG. 1, the collectorcurrent of the transistor Q2 (i.e., the current output from the firstoutput terminal OUT1 is n(I+i), whereas the collector current of thetransistor Q4 (i.e., the current output from the second output terminalOUT2) is n(I-i).

Therefore, the current gain Gi is:

    Gi=n                                                       (1)

The current gain Gi is no more than the current-mirror ratio n, i.e.,the ratio of the emitter area of the transistor Q2 to that of thetransistor Q1 or the ratio of the emitter area of the transistor Q4 tothat of the transistor Q3.

FIG. 2 shows a modification of the differential current amplifiercircuit of FIG. 1. This amplifier circuit is different from the circuitof FIG. 1 in that it has an input-signal current source and transistorsQ5 and Q6. The current source supplies a differential input-signalcurrent consisting of a DC component I and a signal component (either +ior -i) superposed on the DC component I. Since the circuit is identicalto the circuit of FIG. 1 in all other respects, the same symbols andnumerals are used to designate the same components as those illustratedin FIG. 1. The input-signal current source comprises PNP transistors Q01and Q02 and a constant current source 60. The NPN transistors Q01 andQ02 have their emitters connected together and, thus, constitute adifferential pair. The constant current source 60 is connected betweenthe emitter node of the transistors Q01 and Q02 and a Vcc-currentpotential, and supplies a constant current 2I. The collectors of thetransistors Q01 and Q02 are connected to the collectors of thetransistors Q1 and Q3, respectively. Hence, when input voltages INA andINB, either containing a differential signal component, are applied tothe bases of the transistors Q01 and Q02, signal currents (I +i) and(I-i) flow to the transistors Q1 and Q3, respectively. The PNPtransistors Q5 and Q6 are connected together in mirror-current fashion.Their emitters are coupled to the Vcc potential, their bases areconnected to each other, and their collectors are connected to thecollectors of the transistors Q2 and Q4. The base and collector of thetransistor Q6 are connected together. The collector of the transistor Q5is coupled to an output terminal OUT. A load resistor RL is connectedbetween the output terminal OUT and the ground GND.

In the differential current amplifier circuit of FIG. 2, the outputcurrent (I-i) of the transistor Q4 is subtracted from the output current(I+i) of the transistor Q2 by means of the third current mirror circuit,thereby extracting a signal component. Hence, the current i_(out) by theamplifier circuit is 2ni. In other words, the current gain Gi of thecircuit is:

    Gi=2n                                                      (2).

Obviously, this current gain is twice the current gain Gi of thedifferential current amplifier circuit illustrated in FIG. 1.

In the differential current amplifier circuit of FIG. 2, however, it isnecessary to used more and more current-mirror stages in order toincrease the current gain Gi. The circuit needs to have more elements,and inevitably consume more power. If the current-mirror ratio (i.e.,the emitter-area ratio) is increased, each transistor must be larger,and the integrated circuit chip must be proportionally larger.Consequently, the cost of the integrated circuit will increase, and thepower consumption of the circuit will increase, too.

As has been described, the conventional differential current amplifiercircuit needs more elements or must comprise large-sized transistors, inorder to have a greater current gain. The conventional circuitinevitably consumes much power, and must be made of a large, high-costintegrated-circuit chip.

The present invention has been made to solve the problems describedabove. Its object is to provide a differential current amplifier circuitwhich is relatively simple in structure, needs no additional DC currentsource, and can notwithstanding obtain a great current gain, requires arelatively small number of elements, consumes but a little power, andcan hence be made of a low cost integrated circuit.

DISCLOSURE OF THE INVENTION

A differential current amplifier circuit according to the presentinvention has first to sixth bipolar transistors of the same polarity,and first and second resistors. The first and second bipolar transistorshave their emitters connected to each other. The first and secondresistors are coupled between the collector-base path of the firstbipolar transistor and that of the second bipolar transistor. The baseand collector of the third bipolar transistor are connected together,and the collector-emitter path thereof is connected between the emitternode of the first and second bipolar transistors and a predeterminedpotential terminal. The base and emitter of the fourth bipolartransistor are connected to the base and emitter of the third bipolartransistor, respectively. The fifth and sixth bipolar transistors havetheir emitters connected to each other, the node of which is coupled tothe collector of the fourth bipolar transistor. The bases of the fifthand sixth bipolar transistors are connected to the collectors of thesecond and first bipolar transistors, respectively. The collectors ofthe fifth and sixth bipolar transistors are coupled to the a secondoutput terminal and a first output terminal, respectively. The first andsecond bipolar transistors have their bases connected together, andtheir collectors connected to a first signal input terminal and a secondsignal input terminal, respectively. Alternatively, the first and secondbipolar transistors have their collectors connected together, and theirbases connected to the first and second signal input terminals,respectively.

Assuming that the first and second resistors have the same resistance R,the base node of or collector node of the first and second transistorscan be regarded as ground potential point in the case where an inputsignal current (I+i) obtained by superposing a signal component on theDC component is input to the first signal input terminal, and an inputsignal current (I-i) obtained by superposing a signal component (-i),which is differential to the signal component (+i), on the DC componentis input to the second signal input terminal. The signal component iflows to the first and second resistors, and flows to neither the firsttransistor nor the second transistor. In other words, the DC current 2I,superposed with no signal currents and hence clean, flows to the thirdtransistor. Hence, a DC current 2n·I (where n is the current-mirrorratio, or the emitterarea ratio) flows to the fourth transistor coupledto the third transistor in current-mirror fashion. Since a DC currentn.I flows to the fifth and sixth transistors, the AC emitter resistancere of these transistors is:

    re=VT/(n·I) (VT is thermal voltage).

The signal current component i₂ flowing to the fifth and sixthtransistors is:

    i.sub.2 =R·i/re.

Therefore, the current gain Gi is:

    Gi=i.sub.2 =n·I·R/VT.

As has been described, a DC current, which is superposed with no signalcurrents and is hence clean, flows to the third and fourth transistors.Therefore, the circuit requires no DC current source for thesetransistors. The circuit is relatively simple in structure, requiring asmall number of elements, consumes but a little power, and can be madeof a low-cost integrated circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram showing a conventional differential currentamplifier circuit.

FIG. 2 is a circuit diagram showing another conventional differentialcurrent amplifier circuit.

FIG. 3 is a circuit diagram illustrating a differential amplifiercircuit according to an embodiment of the present invention.

FIG. 4 is a circuit diagram showing a modification of the differentialcurrent amplifier circuit shown in FIG. 3.

FIG. 5 is a circuit diagram showing a differential current amplifiercircuit according to another embodiment of the invention.

FIG. 6 is a circuit diagram illustrating a modification of thedifferential current amplifier circuit shown in FIG. 5.

BEST MODES OF CARRYING OUT THE INVENTION

An embodiment of the present invention will now be described, withreference to the accompanying drawings.

FIG. 3 illustrates a differential current amplifier circuit according toa first embodiment of the present invention, which is formed in asemiconductor integrated circuit. As is shown in FIG. 3, the amplifiercircuit comprises first and second NPN transistors Q1 and Q2, theemitters of which are connected to each other and the bases of which areconnected to each other. A resistor R1 is connected between thecollector of the transistor Q1 and the base node of the transistors Q1and Q2. A resistor R2 is connected between the collector of thetransistor Q2 and the base node of the transistors Q1 and Q2. Thecollectors of the transistors Q1 and Q2 are coupled to a first signalinput terminal IN1 and a second signal input terminal IN2, respectively.

A third NPN transistor Q3 has its collector-emitter path connectedbetween the emitter node of the transistors Q1 and Q2 and the lowpotential (i.e., the ground GND in this embodiment), and has itscollector and base connected to each other (that is, diode-connected). Afourth NPN transistor Q4 has an emitter area which is n times greaterthan that of the transistor Q3, and is connected to the transistor Q3 incurrent-mirror fashion (that is, its base and emitter are connected tothe base and emitter of the transistor Q3, respectively).

A fifth NPN transistors Q5 and a sixth NPN transistor Q6 have theiremitters connected to each other, thus forming a differential pair. Theemitter node of these transistor Q5 and Q6 is connected to the collectorof the transistor Q4. Their bases are connected to the collectors of thetransistors Q1 and Q2, respectively, The collector of the transistor Q6is connected to a first output terminal OUT1, and the collector of thetransistor Q5 is coupled to the second output terminal OUT2.

Let us assume that in the differential current amplifier circuit shownin FIG. 3, the transistors Q1 and Q2 have substantially identicalcharacteristics, and that the resistors R1 and R2 have the sameresistance. Then, the base node of the transistors Q1 and Q2 is regardedas a ground potential point when the first signal input terminal IN1receives an input signal current (I+i) consisting of a DC component Iand a signal component (+i) superposed on the DC component I, and thesecond input signal terminal IN2 receives an input signal current (I-i)consisting of a DC component I and a signal component (-i) superposed onthe DC component I and differential to the signal component (+i). Inthis case, the signal current component i₁ (=i) flows to the resistorsR1 and R2, and flows to neither the transistor Q nor the transistor Q2.In other words, a DC current 2I, which is superposed with signalcurrents and is therefore clean, flows to the transistor Q3. Hence, a DCcurrent 2n·I flows to the transistor Q4 connected to the transistor Q3in current-mirror fashion at the ratio of 1:n.

A DC current of n·I flows to the transistors Q5 and Q6. Hence, the ACemitter resistances re (i.e., re₅ and re₆) of the transistors Q5 and Q6are:

    re=re.sub.5 =re.sub.6 =VT/n·I

where VT is a thermal voltage which is 26 mV at 300° K.

A signal current component i₂ flows to the transistors Q5 and Q6.Therefore:

    i.sub.1 ·R =re·i.sub.2.

Hence:

    i.sub.2 =R·i.sub.l /re.

Therefore, the current gain Gi is:

    Gi=i.sub.2 /i.sub.l =n·I·R/VT            (3).

Gi in equation (3) is I·R/VT times greater than the current gain Gi ofthe differential current amplifier circuit illustrated in FIG. 1, whichis defined by equation (1).

Assuming that n=1, I=100 μA, R=10kΩ, then:

    Gi=38.5 (=31.7 dB).

In this case, Gi is about 38.5 times greater than the current gain Gi ofthe differential current amplifier circuit shown in FIG. 1.

FIG. 4 shows a modification of the differential current amplifiercircuit illustrated in FIG. 3. This amplifier circuit is different fromthe circuit of FIG. 3 in two respects. First, it has an input-signalcurrent source. Second, it has transistors Q7 and Q8. The circuit isidentical to the circuit of FIG. 3 in all other respects, and the samesymbols and numerals are therefore used to designate the same componentsas those illustrated in FIG. 3. The current source supplies adifferential input-signal current consisting of a DC component I and asignal component (either +i or -i) superposed on the DC component I. Theinput-signal current source comprises PNP transistors Q01 and Q02 and aconstant current source 20. The NPN transistors Q01 and Q02 have theiremitters connected together and, thus, constitute a differential pair.The constant current source 20 is connected between the emitter node ofthe transistors Q01 and Q02 and a high power-supply potential (i.e., theVcc-power potential in this embodiment), and supplies a constant current2I. The collectors of the transistors Q01 and Q02 are connected to thecollectors of the transistors Q1 and Q2, respectively. Hence, when inputvoltages INA and INB, either containing a differential signal component,are applied to the bases of the transistors Q01 and Q02, signal currents(I+i) and (I-i) flow to the transistors Q1 and Q2, respectively. The PNPtransistors Q7 and Q8 are connected together in mirror-current fashion.Their emitters are coupled to the Vcc potential, their bases areconnected to each other, and their collectors are connected to thecollectors of the transistors Q5 and Q6. The base and collector of thetransistor Q7 are connected together. The collector of the transistor Q8is coupled to an output terminal OUT. A load resistor RL is connectedbetween the output terminal OUT and the ground GND.

The differential current amplifier circuit of FIG. 4 operates, basicallyin the same way as the circuit of FIG. 3. However, the current n(I-i₂)of the transistor Q5 is subtracted from the current n(I+i₂) of thetransistor Q6 by means of the transistors Q7 and Q8, thereby extractinga signal component. The current i_(out) output by the amplifier circuitis 2i₂. Therefore, the current gain Gi of the circuit is: ##EQU1##

Gi in equation (4) is i·R/VT times greater than the current gain Gi ofthe differential current amplifier circuit illustrated in FIG. 2, whichis defined by equation (2).

Assuming that n=1, I=100 μA, R=10kΩ, Gi of the conventional circuit ofFIG. 2, which is defined by equation (2), is:

    Gi=2 (6 dB).

By contrast, Gi of the present embodiment (FIG. 4) is: ##EQU2## In thiscase, Gi is about 38.5 times greater than the current gain Gi of thedifferential current amplifier circuit shown in FIG. 2

If the current-mirror ratio n is set at 38.5 in order to obtain agreater current gain Gi in the differential current amplifier circuit ofFIG. 2, the circuit will require transistors of greater size, and willinevitably made of an integrated circuit chip which has a greater sizeand, hence, a higher cost, and consumes more power.

FIG. 5 illustrates a differential current amplifier circuit according toa second embodiment of the present invention. This amplifier circuit isdifferent from the circuit of FIG. 3 in three respects only. First, theemitters of the first and second NPN transistors Q1 and Q2 are connectedto each other, the collectors thereof are coupled to each other, and thebases thereof are connected to the first and second signal inputterminals IN1 and IN2, respectively. Second, the first resistor R1 iscoupled between the base of the first NPN transis tor Q1 and thecollector node of the NPN transistors Q1 and Q2. Third, the secondresistor R2 is connected between the base of the second NPN transistorQ2 and the collector node of the NPN transistors Q1 and Q2. In any otherrespect, the circuit of FIG. 5 is identical to that one shown in FIG. 3,and the same symbols and numerals are therefore used to designate thesame components as those illustrated in FIG. 3.

The differential current amplifier circuit of FIG. 5 operates, basicallyin the same way as the circuit of FIG. 3. Therefore, its current gain Giis:

    Gi=i.sub.2 /i.sub.l =n·I·R/VT            (5).

Obviously, equation (5) is exactly the same as equation (3). It followsthat the current gain Gi of the amplifier circuit of FIG. 5 is equal tothat of the amplifier circuit illustrated in FIG. 3.

FIG. 6 illustrates a modification of the differential current amplifiercircuit shown in FIG. 5. This circuit is identical to the circuit ofFIG. 5, except for two respects. First, it has an input-signal currentsource. Second, it has NPN transistors Q7 and Q8. The circuit isidentical to the circuit of FIG. 5 in all other respects, and the samesymbols and numerals are therefore used to designate the same componentsas those illustrated in FIG. 5. The current source supplies adifferential input-signal current consisting of a DC component I and asignal component (either +i or -i) superposed on the DC component I.

The input-signal current source comprises PNP transistors Q01 and Q02and a constant current source 40. The NPN transistors Q7 and Q8 arearranged in the same way as in the circuit shown in FIG. 4. Thecollector of the transistor Q8 is connected to an output terminal OUT. Aload resistor RL is connected between the output terminal OUT and theground GND.

The differential current amplifier circuit of FIG. 6 operates in thesame way as the circuit of FIG. 4. The current n(I-i₂) of the transistorQ5 is subtracted from the current n(I+i₂) of the transistor Q6 by meansof the transistors Q7 and Q8, thereby extracting a signal component. Thecurrent i_(out) output by the amplifier circuit is 2i₂. Therefore, thecurrent gain Gi of the circuit is: ##EQU3## Obviously, equation (6) isexactly the same as equation (4). Hence, the current gain Gi of theamplifier circuit of FIG. 6 is equal to that of the amplifier circuitillustrated in FIG. 4.

In the embodiments and the modifications thereof, all described above,the Vcc power-supply potential can be replaced by the ground potential,and the ground potential can be replaced by a negative power potential.Further, the high power-supply potential and the low power-supplypotential can be replaced by each other, and the PNP transistors can bereplaced by the NPN transistors, and vice versa.

As has been described above, the differential current amplifier circuitaccording to the present invention is relatively simple in structure,requires no additional DC current source, and can notwithstanding obtaina great current gain, requires a relatively small number of elements,consumes but a little power, and can hence be made of a low-costintegrated circuit.

INDUSTRIAL APPLICABILITY

The differential current amplifier circuit according to the invention isuseful in the case where an amplifier circuit is required whichcomprises a reactively small number of elements, consumes but a littlepower, and can notwithstanding can obtain a great current gain.

We claim:
 1. A differential current amplifier circuit comprising firstto sixth bipolar transistors of the same polarity, first and secondresistors, characterized in that:said first and second bipolartransistors have their emitters connected to each other; said first andsecond resistors are coupled between the collector-base path of saidfirst bipolar transistor and that of said second bipolar transistor thebase and collector of said third bipolar transistor are connectedtogether, and the collector-emitter path thereof is connected betweenthe emitter node of the first and second bipolar transistors and apredetermined potential terminal; the base and emitter of said fourthbipolar transistor are connected to the base and emitter of said thirdbipolar transistor, respectively; said fifth and sixth bipolartransistors have their emitters connected to each other, the node ofwhich is coupled to the collector of the fourth bipolar transistor; thebases of said fifth and sixth bipolar transistors are connected to thethe collectors of said second and first bipolar transistors,respectively; the collectors of said fifth and sixth bipolar transistorsare coupled to the a second output terminal and a first output terminal,respectively; and said first and second bipolar transistors have theirbases connected together, and their collectors connected to a firstsignal input terminal and a second signal input terminal, respectively,or have their collectors connected together, and their bases connectedto the first and second signal input terminals, respectively.
 2. Thedifferential current amplifier circuit according to claim 1,characterized in that said first to sixth bipolar transistors are NPNtransistors, said first and second bipolar transistors havesubstantially the same characteristic, said first and second resistorshave substantially the same resistance, and said predetermined potentialterminal is a ground-potential terminal.